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Deterministic chaos reproduces the randomness of living microswimmers

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Several microorganisms (bacteria, algae...) explore the surrounding space in search of nutrients by following a linear walk for a period of time (called a "run") followed by a sudden change in direction (called a "tumble") and so on. This type of trajectory is known as a "run-and-tumble". Over a fairly long period of time (recording several tens or hundreds of runs and tumbles), the trajectory looks like a random walk, allowing the space to be covered efficiently in search of food. The alternation between run and tumble is a biochemically regulated phenomenon using receptors (proteins) anchored in the cytoplasmic membrane of the microorganism. Microorganisms have had to develop a complex biochemical strategy that allows them to navigate more efficiently in space than by using Brownian motion alone.

The theoretical work carried out at the Liphy shows that artificial swimmers (phoretic particles) can autonomously perform the same type of run and tumble movement as living microorganisms by evoking relatively simple but robust physical mechanisms only. It is shown that this type of trajectory is the result of instability of the straight trajectory, which is based on non-linear effects, which are at the origin of the particle’s propulsion. The movement of these artificial swimmers, lacking any biological regulation, mimics with remarkable similarity the movement of real organisms. This seemingly random movement results from a transition to chaos of purely deterministic origin, i.e. without probabilistic ingredients. This result shows that it is possible to allow artificial swimmers to explore space with minimal ingredients without resorting to a complex biochemical strategy. It also provides a detailed understanding of artificial microswimmers that may in the future be used to perform more or less complex nanorobotic tasks, such as targeted therapy, microsurgery or environmental sanitation.

View online : Chaotic Swimming of Phoretic Particles Wei-Fan Hu, Te-Sheng Lin, Salima Rafai, and Chaouqi Misbah Phys. Rev. Lett. 2019